Post-excavation corrosion remains a significant consideration in the stabilization and preservation of archeological artifacts. A major consideration is removal of chloride ion, a major factor in the corrosive process.
Corrosion in its simplest form is deterioration of metal by oxidation and is commonly recognized as rusting, expressed chemically as4Fe(solid)+3O2(gas)→2Fe2O3.H2(solid)
The process of corrosion or rust can be explained in terms of electrochemical mechanisms in which a water droplet constitutes a minute galvanic cell with defined anode and cathode regions. Electrons flow from the anode to the cathode through the metal, and ions flow through the water droplets. As one of average skill in the art recognizes, this electrochemical model provides a clear explanation as to why the presence of dissolved salts (particularly the Cl−1 ion) accelerates corrosive processes.
Corrosive processes are a widely acknowledged nemesis of archeologists, particularly those dedicated to finding and preserving marine artifact. In recent decades, technology to locate, identify, and recover historic remains of ships and aircraft lost in combat, by accident, or as a result of storms has exploded. The discovery of the remains of the “Titanic” in the North Atlantic Ocean in September, 1988, 73 years after she sank in about 12,500 feet of water, and the subsequent recovery of about 1800 artifacts from the wreckage ignited interest in the recovery/preservation of marine artifacts. The hull of the Titanic showed expected devastation from corrosive forces; much of the recovered artifact was porcelain and material not as subject to such forces. None-the-less, recovery of iron artifact that is highly susceptible to corrosion is of continuing interest.
In May, 1995, after decades of futile searching, a small group of divers located the hull of the Confederate submarine, “H. L. Hunley,” in less than 40 feet of water in the Atlantic Ocean, immediately outside of the Charleston, S.C. harbor. In February 1864, following a historic, successful attack on the Union ship “Housatonic,” the “Hunley” and her full crew were lost.
The hull of the “Hunley” was, as expected, badly corroded and fragile. See FIG. 1. In order to preserve the priceless, historic, iron artifact, devices and methods to minimize continued losses and to preserve and stabilize the recovered hull and associated artifact became of immediate concern. Prior art addresses corrosion prevention, but fails to address preservation and stabilization of iron artifact.
For example, U.S. Pat. No. 2,490,062 issued Dec. 6, 1949 to Jernstedt was, “to provide for producing protective coatings on metal surfaces ***”. The metal surfaces included iron, zinc, and aluminum. The coatings were to protect against rust formation, not to stabilize rusted-artifact.
Later, U.S. Pat. No. 3,961,991 issued Jun. 8, 1976 to Yoshida and Takagi indirectly addressed corrosion prevention in describing a pretreatment of large metal surfaces, such as the hull of a ship, to prepare the surface for coating with a rust inhibiting coating. Again, the emphasis is on prevention.
In at least one patent the marine environment is recognized as a major cause of corrosion. U.S. Pat. No. 4,844,865 issued Jul. 4, 1989 to Shimada and Sakakibara recognized salt water itself, and corrosion caused by sea salt particles as the principal causes of the decay of steel structures. The '865 patent relates to “non-magnet steel material” for use in magnetic floating high-speed railways nuclear fusion facilities, and similar applications. In the '865 patent, the issue of corrosion resistance is addressed by the development of special materials, not by prevention as described above or by stabilization of artifact.
U.S. Pat. No. 4,950,453 issued to Murray on Aug. 21, 1990 discloses and claims a method of passively forming a layer of zinc silicate-on metal surfaces to protect the surfaces from corrosion. The metal surface to be protected is pretreated with a very dilute solution of a soluble zinc salt and ultimately the treatment leads to the formation of a protective coating of zinc silicate. U.S. Pat. No. 6,468,364 issued Oct. 22, 2002 to Marecic describes an anticorrosion treatment utilizing a zinc/aluminum alloy coating with a molybdenum/phosphoric acid solution.
U.S. Pat. No. 5,849,220 issued Dec. 15, 1998 to Batton and Chen provides a corrosion inhibiting composition adapted for use in both aqueous and non-aqueous fluid involving both ferrous-containing and non-ferrous containing metals. The composition consists of two surfactants, a sorbitan fatty acid ester and a polyethylene derivative. Although markedly different from the '453 patent, like the '453 patent, the '220 patent depends on a specific protective layer to prevent corrosion.
Two related problems are addressed by U.S. Pat. No. 6,425,997 issued Jul. 30, 2002 to Johnson: removal of chloride ion from a corroded, pitted steel surface and measuring the amount of chloride present by use of a conductivity cell. The process claimed in the '997 patent to remove water soluble ions comprises placing a corroded surface in contact with deionized water while passing a high frequency waveform alternating current between steel anodes and the corroded metal surface (which serves as the cathode).
The preceding summary of U.S. patents is a representative summary of prior art that relates to corrosion prevention or management. Clearly, the major focus is on corrosion prevention of “new” material employing some type of protective coating. The '997 patent taught the significance of removal of chloride ions only from the use of electrolysis in highly specialized electrical conditions.
Interest continues to grow in preservation and stabilization of artifact recovered from marine environment and there remains room for innovation and improvement in methods and devices to preserve and stabilize such artifact by rapid and thorough removal of chloride ions.